1. Field
Exemplary embodiments of the present invention relate to a semiconductor fabrication technology, and more particularly, to a semiconductor device having an air gap between bit lines and a method for fabricating the same.
2. Description of the Related Art
With the rapid development of a semiconductor technology and the high density requirements, various types of technologies for bonding and stacking wafers have been developed. According to these technologies, a new wafer is bonded on a wafer substrate having elements, and other elements are formed subsequently.
Recently, there has been employed a technology of bonding a wafer, in which N/P/N/P layers have been stacked, on a metalized wafer, patterning the NPN layers in a subsequent process, and forming elements.
In order to form the wafer in which the N/P/N/P layers have been stacked, there is a method for ion implanting N-type impurities and P-type impurities into a substrate at different depths. However, in the case of forming the N/P/N/P layers using only ion implantation, ion implantation energy and a dose amount are increased substantially in order to form impurity layers at the lower portion of the substrate. Furthermore, in heat treatment for activating dopants after the ion implantation, impurities between layers are diffused, so that the N/P/N/P layers are not distinguished from one another.
In order to solve the concern, there has been proposed a method for forming two of the N/P/N/P layers by ion-implanting impurities into the substrate and forming the other two layers through single crystalline growth. However, this method has concerns that processes are complicated and process cost is high. Furthermore, since a growth thickness of a single crystalline layer should be thick, the method is disadvantageous in terms of cost, and does not completely solve the concerns due to the ion implantation.
Meanwhile, with an increase in the degree of integration of elements, parasitic capacitance between bit lines is increased, so that the reliability of the elements is reduced.